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ORIGINAL PAPER
Grain size characteristics of superficial sediments of the Gulfof Tunis (NE Tunisia)
Hanen Saïdi & Radhia Souissi & Fouad Zargouni
Received: 28 December 2012 /Accepted: 10 June 2013# Saudi Society for Geosciences 2013
Abstract This work presents a recent grain size study char-acterizing superficial sediments of the whole sectors of theGulf of Tunis which extends from Sidi Ali El Mekki to SidiDaoued. In this study, a total of 112 samples were collectedfrom different depths following 26 profiles perpendicular tothe shore and from rivers. The determination of the propor-tions of coarse and fine fractions and the interpretation of thegrain size results lead to dividing the Gulf of Tunis into twoprincipal coasts: (1) the western coast situated between SidiAli El Mekki and Borj Cédria, which is characterized bythree sedimentary facies: (a) a sandy facies along the coast-line, (b) a mixed facies between Ghar El Melh and Raoued,near the Medjerda River, and between La Goulette andEzzahra, near the Miliane River, and (c) a muddy faciesbetween Ghar El Melh and Kalâat El Andalous and coveredby fine to very fine sands, and (2) the eastern coast, which ischaracterized by a sandy facies and covered by medium tocoarse sands. The dominant N–E and N–W waves generatelongshore currents that provide sediment transport followingthe directions N–S, from Sidi Ali El Mekki to Kalâat ElAndalous; SE–NW, from Gammarth to Raoued; NE–SW,from Sidi Bou Saïd to La Goulette and from Sidi Daoued
to Bir El Jadi; SE–NW, from Hammam-Plage to Ezzahra;and SW–NE, from Port aux Princes to Rtiba.
Keywords Superficial sediments . Grain size . Sedimenttransport . Gulf of Tunis . Tunisia
Introduction
The coastal area is, generally, the center of several activitiessuch as industry, commerce, fishing, navigation, and leisure.It is considered as a big economic and strategic patrimonydespite having suffered from erosion in many parts of theworld (Mignot 1988).
This dynamic area attracts researchers to understand anddiscover different complex coastal processes that left theirimprints in sediments (Angusamy and Rajamanickam 2007).Several researchers like Bascom (1951), Folk and Word(1957), Inman and Chamberlain (1955), Friedman (1961,1979), Passega (1964), Sahu (1964), Ottmann (1965), Vatan(1967), Moiola and Weiser (1968), Visher (1969), Solohuband Klovan (1970), Rivière (1977), Vincent (1986, 1998),Barusseau et al. (1999), Chamley (1987, 2000), Klein et al.(2004), Lancker et al. (2004), Horn and Walton (2007),Anthony and Hequette (2007), Kathirkaman and Krishnamur-thy (2010), and Barusseau (2011) have used grain size data tocharacterize sediments and interpret their transport and theirdifferent environments of deposition.
Many sectors of the Gulf of Tunis, which is situatedin the northeast of Tunisia, suffered from erosion andfragility (Nouri 1979; Kouki 1984; Bouhafa 1985; Sliti1990; Oueslati 1993, 1994, 2004; El Arrim 1996; ZeggafTahri 1999; Saïdi 2004).
The ability of superficial sediments to be transported anddeposed is related with their grain size characteristics. So,grain size study of the superficial sediments of the Gulf ofTunis was necessary. It was carried out by Kouki (1984) whostudied the sector situated between La Goulette and Sidi
H. Saïdi (*) : F. ZargouniFaculty of Sciences of Tunis, Campus Universitaire El-Manar,2092 Tunis, Tunisiae-mail: [email protected]
H. Saïdie-mail: [email protected]
F. Zargounie-mail: [email protected]
R. SouissiNational Institute of Research and Chemical and Physical Analysis,Sidi Thabet, Tunisiae-mail: [email protected]
Arab J GeosciDOI 10.1007/s12517-013-1008-0
Fig. 1 Location map and geological setting of the Gulf of Tunis
Fig. 2 Bathymetry (edited in2000) and hydrographic systemof the Gulf of Tunis
Arab J Geosci
Erraïs, El Arrim (1996) who focalized his study on the zonelocated between Ghar El Melh and Sidi Erraïs, and Addedet al. (2003) who studied the grain size of superficial sedi-ments of the Gulf of Tunis which were collected from depthsexceeding −10 m.
With the exception of those researchers, there are nopreviously published studies on the superficial sedimentsof the entire sectors which formed the Gulf of Tunisand which extended from Sidi Ali El Mekki to SidiDaoued. So, the present work aims to redress this situ-ation by characterizing superficial sediments of all thesectors of the Gulf of Tunis. Adding to that, it presentsthe direction of the sediments transport along the stud-ied area.
Regional setting of the Gulf of Tunis
Geographical location and geology
The Gulf of Tunis is situated in the northeastern part ofTunisia, between the latitudes 36°41′24″ and 37°12′0″ andthe longitudes 10°07′12″ and 10°55′12″. It extends from SidiAli El Mekki to Sidi Daoued over a distance of 150 km. It islimited on the north by the Mediterranean Sea, on the southby the regions of Mornag and Grombalia, on the west by theregions of Soukra, Ariana, and Tunis, and on the east by theregions of Kélibia, Menzel Témime, and Korba (Fig. 1).
The Gulf of Tunis is a large rift valley formed during thePleistocene (Azouz 1973). Its periphery shows the arrange-
Table 1 Occurrence (in percent)of waves in the Gulf of Tunisbasing on their directions andtheir heights, during the period2002–2004 (Gasdaoui 2005)
Height (m) Direction
W NNW NW N NE E SE S SW Total (%)
1 0.45 0.45 5.84 4.04 12.12 1.8 5.98 0.89 4.64 36.22
2 6.44 0 9.43 7.63 8.83 2.84 3.44 0 2.84 41.46
3 1.2 0 4.94 5.39 2.39 0 2.39 0 0.3 16.61
4 0.9 0 0 0.45 1.64 0 0.3 0 0 3.29
5 0 0 0 0 0 0 0.15 0 0 0.150
6 0.15 0 0 0 0.15 0 0 0 0 0.3
7 0 0 0 0 0 0 0.6 0 0 0.6
8 0 0 0 0 0 0 0.15 0 0 0.15
Total (%) 9.13 0.45 20.21 17.51 25.15 4.64 13.02 0.89 7.78 98.9
Fig. 3 Occurrence (in percent)of waves in the Gulf of Tunisbasing on their directions (a)and their heights (b), during theperiod 2002–2004
Arab J Geosci
Fig. 4 Propagation of waves coming from the northeast sector, between Sidi Ali El Mekki and Kalâat El Andalous (a), between Kalâat El Andalousand Raoued (b), and between Raoued and Gammarth (c)
Arab J Geosci
Fig. 5 Circulation of currents generated by waves coming from the northeast sector, between Sidi Ali El Mekki and Kalâat El Andalous (a), betweenKalâat El Andalous and Raoued (b), and between Raoued and Gammarth (c)
Arab J Geosci
Fig. 6 Propagation of wavescoming from the northeastsector between Cap Gammarthand Cap Carthage (a) andcirculation of generated currents(b)
Fig. 7 Propagation of wavescoming from the northeastsector between Sidi Bou Saïdand La Goulette (a) andcirculation of generated currents(b)
Arab J Geosci
ment of the compressive structures, such as the anticlinal ofGhar El Melh, the anticlinal of Jebel Abderramen in CapBon, and the monoclinals of Gammarth, Sidi Bou Saïd, andAmilcar and the complex structures of Jebel Bou Kornine
and Jebel Korbous, and the depressive structures, such as thegraben of Ariana and Sebkhet Sejoumi and the ditches ofMornag and Grombalia. The Gulf is affected by structures ofEW, NW–SE, and NE–SW directions (Fig. 1). It is borderedby secondary, tertiary, and quaternary surroundings.
Geomorphology and hydrographic system
The beaches of the Gulf of Tunis are interrupted by naturalbarriers, such as the mountain of Cap Sidi Ali El Mekki, thecliffs of Gammarth and Sidi Bou Saïd, and the mountain ofKorbous, and by artificial constructions, such as yacht harborsand marinas (fishing harbors of Ghar El Melh, Kalâat ElAndalous, La Goulette, and Sidi Daoued and the pleasure portof Sidi Bou Saïd) and protective structures (groins implantedbetween Salammbô and La Goulette and breakwaters con-structed between La Goulette and Soliman).
The seabeds of the Gulf of Tunis are not regular. In thewesterncoast located between Sidi Ali El Mekki and Borj Cédria, theisobaths are very close to each other near the rocky point of SidiAli El Mekki and near the cliffs of Gammarth and Sidi Bou Saïd,reflecting the existence of very steep slopes, and they are spaced atberries, indicating that the slopes are gentle. In the eastern coastsituated betweenBorj Cédria and Sidi Daoued, the slopes are verysteep (Fig. 2). The hydrographic system of the Gulf of Tunis iscontrolled by the two important rivers of Medjerda and Milianeand the rivers of Soltane, Bizirk, and Abid (Fig. 2).
Waves and current dynamic status
According to the annual statistics for 2002–2004 (Gasdaoui2005), the dominant waves come from northeast and
Fig. 8 Propagation of wavescoming from the northeastsector between Radès andEzzahra (a) and circulation ofgenerated currents (b)
Fig. 9 Propagation of waves coming from the northeast sector betweenHammam-Lif and Hammam-Plage (a) and circulation of generatedcurrents (b)
Arab J Geosci
northwest sectors. The most occurring waves have heights of1 and 2 m (Table 1; Fig. 3).
The coastal dynamics of the Gulf of Tunis were studiedusing the Coastal Modeling System (SMC). This numericalmodel represents a Coastal Management System developedby the Coast General Director (DGC) and the Coastal Re-search Group (GIOC) of the University of Cantabria. Theinput data used in the SMC model are bathymetric andhydrodynamic data such as waves.
The dominant waves coming from the northeastern sector(Fig. 4a–c) generate longshore currents of north–south di-rection, from Sidi Ali El Mekki to Gammarth (Fig. 5a–c).These currents provide alongshore sediment transport, espe-cially behind the Sand Spit of Kalâat El Andalous. Thesewaves also create gyratory currents downstream the SandSpit of Kalâat El Andalous (Fig. 5a) and swirling currentsbetween Raoued and Gammarth, very near the coastline.They concentrate behind Cap Gammarth and generate twobranches of longshore currents having opposite directions(Figs. 5c and 6). The first branch is directed to Raoued along
a SE–NW direction. The second branch is directed to LaMarsa along a NE–SW direction.
The dominant waves coming from the northeastern sectorgenerate gyratory currents upstream Kobbet El Hwa of LaMarsa. They concentrate when they strike the cliff of CapCarthage and generate two longshore currents of oppositedirections: a SE–NW current directed to LaMarsa and a NE–SW current directed to Sidi Bou Saïd (Fig. 6b).
The dominant N–E waves generate longshore currentshaving a NE–SW direction, from Sidi Bou Saïd to the fishingharbor of La Goulette (Fig. 7). They change their directiondownstream of the fishing harbor of La Goulette and gener-ate gyratory currents at the coastline of Radès and longshorecurrents having a SE–NW direction, from Hammam-Lif toEzzahra (Fig. 8). At the coastline situated betweenHammam-Lif and Hammam-Plage, the dominant N–Ewaves generate longshore currents having a SE–NW direc-tion (Fig. 9).
The dominant waves coming from the northwestern sec-tor attack the coastline situated between Borj Sidi Cédria and
Fig. 10 Propagation of wavescoming from the northwestsector between Borj Cédria andSidi Erraïs (a) and circulation ofgenerated currents (b)
Arab J Geosci
Sidi Erraïs and generate currents of high energy and gyratorycurrents (Fig. 10), leading to the mobilization of sediments.
Between Port aux Princes and Sidi Daoued, the dominantnortheastern waves generate currents which become stronger atthe approach of the shoreline (Fig. 11). They also generatecurrents of longshore drift having a SW–NE direction, betweenPort aux Princes and Rtiba, and a NE–SW direction, from BirEl Jadi to Sidi Daoued. Between Bir El Jadi and Rtiba, we note
the presence of two currents having opposite directions andsometimes the presence of rip currents (Fig. 11).
Transport of sediment
Sediment transport is very important to explain erosion oraccretion processes. It takes place in nearshore environmentsby waves and generated currents. The bed load grain size is
Fig. 11 Propagation of wavescoming from the northwestsector between Port aux Princesand Sidi Daoued (a) andcirculation of generated currents(b)
Arab J Geosci
Fig. 12 Location of sampling sites of superficial sediments of the Gulf of Tunis
Table 2 Formulae used for com-puting various grain size parame-ters of Folk and Word (1957)
Grain sizeparameters
Formulae
Mean grain size Mz=(Q16+Q50+Q84)/3
Coarse sands: Mz<1 Ø (Mz>500 μm)
Medium sands: 1 Ø<Mz<2 Ø (250<Mz<500 μm)
Fine sands: 2 Ø<Mz<3 Ø (125<Mz<250 μm)
Very fine sands: 3 Ø<Mz<4 Ø (63<Mz<125 μm)
Standard deviation(sorting index)
σ=(Q84−Q16)/4+(Q95−Q5)/6.6
Very well sorted sands: σ<0.35 Ø
Well sorted sands: 0.35 Ø<σ<0.5 Ø
Moderately sorted sands: 0.5 Ø<σ<1 Ø
Poorly sorted sands: 1 Ø<σ<2 Ø
Very poorly sorted sands: 2 Ø<σ<4 Ø
Skewness Ski=(Q16+Q84−2Q50)/2(Q84−Q16)+(Q5+Q95−2Q50)/2(Q95−Q5)
Very negatively skewed distribution: highly coarse-skewed sediment: −1Ø<Ski<−0.30 Ø
Negatively skewed distribution: coarse-skewed sediment: −0.30 Ø<Ski<−0.10 Ø
Symmetry: −0.10 Ø<Ski<0.10 Ø
Positively skewed distribution: fine-skewed sediment: 0.10 Ø<Ski<0.30 Ø
Very positively skewed distribution: highly fine-skewed sediment: 0.30 Ø<Ski<1 Ø
Arab J Geosci
Tab
le3
Propo
rtions
(inpercent)of
coarse
andfine
fractio
nsin
superficialsedimentscollected
from
theGulfof
Tun
isandfrom
themou
thsof
rivers,d
uringthecampaigns
of20
06–2
007
Sam
ples
CF
FF
Sam
ples
CF
FF
Sam
ples
CF
FF
Sam
ples
CF
FF
R1-0
––
R1-1
93.92
6.08
R1-2
97.2
2.8
R1-3
98.44
1.56
R2-0
99.8
0.2
R2-1
97.23
2.77
R2-2
86.09
13.91
R2-3
60.83
39.17
R3-0
100
0R3-1
93.47
6.53
R3-2
92.5
7.5
R3-3
92.5
7.5
R4-0
100
0R4-1
1.2
98.8
R4-2
94.9
5.1
R4-3
35.53
64.47
R5-0
100
0R5-1
98.04
1.96
R5-2
52.2
47.8
R5-3
24.45
75.55
R6-0
99.87
0.13
R6-1
99.84
0.16
R6-2
93.93
6.07
R6-3
41.01
58.99
R7-0
100
0R7-1
99.77
0.23
R7-2
98.93
1.07
R7-3
40.87
59.13
R8-0
99.98
0.02
R8-1
99.84
0.16
R8-2
99.52
0.48
R8-3
99.47
0.53
R9-0
99.85
0.15
R9-1
99.93
0.07
R9-2
98.64
1.36
R9-3
98.47
0.53
R10
-099
.97
0.02
4R10
-1–
–R10
-299
.73
0.27
R10
-399
.73
0.27
R11-0
100
0R11-1
100
0R11-2
99.89
0.11
R11-3
94.59
5.41
R12
-093
.41
6.59
R12
-1–
–R12
-2–
–R12
-389
.88
10.12
R13
-099
.72
0.28
R13
-198
.72
1.28
R13
-292
.21
7.79
R13
-395
.71
4.29
R14
-010
00
R14
-192
.42
7.57
R14
-293
.58
6.43
R14
-367
.13
32.87
R15
-0–
–R15
-194
.97
5.03
R15
-295
.97
4.03
R15
-394
.03
5.97
R16
-010
00
R16
-171
.25
28.75
R16
-255
.74
44.26
R16
-366
.64
33.36
R17
-010
00
R17
-183
.89
16.11
R17
-275
.53
24.47
R17
-368
.231
.8
R18
-099
.95
0.05
R18
-164
.55
35.45
R18
-273
.39
26.61
R18
-368
.531
.5
R19
-010
00
R19
-197
.31
2.69
R19
-297
.29
2.71
R19
-398
.43
1.57
R20
-010
00
R20
-199
.70.3
R20
-295
.13
4.87
R20
-390
.23
9.77
R21
-010
00
R21
-194
.39
5.61
R21
-298
.56
1.44
R21
-397
.27
2.73
R22
-010
00
R22
-199
.99
0.01
R22
-299
.21
0.79
R22
-310
00
R23
-010
00
R23
-110
00
R23
-210
00
R23
-310
00
R24
-010
00
R24
-110
00
R24
-210
00
R24
-310
00
R25
-010
00
R25
-199
.30.7
R25
-210
00
R25
-310
00
R26
-010
00
R26
-1–
–R26
-2–
–R26
-310
00
O.M
edj(a)
100
0O.Sol(a)
99.41
0.59
O.Ab(a)
100
0
O.M
edj(b)
60.89
39.11
O.Sol(b)
99.57
0.43
O.Ab(b)
100
0
O.M
edj(c)
4.28
95.72
O.Sol(c)
17.87
82.13
O.Ab(c)
98.61
1.39
O.M
il(a)
99.59
0.41
O.Biz(a)
99.46
0.54
O.M
il(b)
99.77
0.23
O.Biz(b)
99.76
0.24
O.M
il(c)
99.82
0.18
O.Biz(c)
99.81
0.19
CFcoarse
fractio
n,FFfine
fractio
n,R1-0to
R26
-0samples
collected
from
0m,R1-1to
R26
-1samples
collected
from
−2m,R1-2to
R26
-2am
ples
collected
from
−5m,R1-3to
R26
-3samples
collected
from
−7m;O.M
edj(a,b,c),O
.Mil(a,b,c),O
.Sol(a,b,c),O.Biz(a,b,c),andO.Ab(a,b,c)
samples
collected
from
themou
thsof
theMedjerda,Miliane,Soltane,B
izirk,
andAbidrivers
Arab J Geosci
related to the characteristics of these hydrodynamic param-eters such as energy and direction and to the geomorphologyof the coastal area.
Methodology
A total of 112 samples of superficial sediments were collect-ed from the small depths (0, −2, −5, and −7 m) of the Gulf ofTunis following 26 profiles perpendicular to the shore (R1 toR26) and from the mouths of the rivers of this gulf (Fig. 12),during February/March 2007. The geographical coordinatesof the sampling sites were obtained using a handheld GPS.
In the laboratory, samples were dried in an oven at atemperature of 50 °C. The dried samples were soaked indistilled water and sieved by a sieve of 63 μm to eliminatethe fine fraction which contains grains having a size >63 μm.The coarse fraction (>63 μm) was dried and sieved by Afnorsieve sets stacked in descending order of size for grain sizeanalysis.
The graphical computational method of Folk and Word(1957) was used in order to calculate grain size statistical
parameters: inclusive graphical mean grain size (Mz), inclu-sive graphical standard deviation (σI), and inclusive graphicskewness (Ski) (Table 2). Statistical analysis of the resultsand the graphical presentation of the data were performedusing Origin and Excel softwares.
Results and discussion
Distribution of sedimentary facies
The distribution of the coarse fraction in superficial sedi-ments of the Gulf of Tunis (Table 3; Fig. 13) shows that:
– the western coastline, which is situated between Sidi AliEl Mekki and Borj Cédria, is covered by a sandy facieshaving a coarse fraction (FG) between 70 and 100 %,along the shoreline, by a mixed or muddy–sandy facies(≤30 to ≤70 % FG) adjacent to the sandy one and locatedbetween Ghar El Melh and Raoued, near the Medjerda,and between La Goulette and Hammam-Lif, around the
Fig. 13 Map of the coarse fraction distribution (in percent) in superficial sediments of the Gulf of Tunis
Arab J Geosci
Tab
le4
Grain
size
parametersof
superficialsedimentscollected
from
theGulfof
Tun
is(0
to−7
m)du
ring
thecampaigns
of20
06–2
007
SMz
σSki
SMz
σSki
SMz
σSki
SMz
σSki
R1-0
––
–R1-1
1.97
90.55
01.97
9R1-2
2.55
10.49
10.011
R1-3
2.62
20.51
4−0.15
8
R2-0
1.93
50.44
7−0.12
8R2-1
2.13
20.61
52.13
2R2-2
3.38
30.38
50.06
1R2-3
3.49
20.40
6−0.27
4
R3-0
2.27
50.64
7−0.09
6R3-1
2.53
90.47
52.53
9R3-2
3.47
90.35
00.06
8R3-3
3.53
30.60
2−0.46
4
R4-0
1.94
90.53
1−0.26
8R4-1
2.87
10.76
62.87
1R4-2
2.90
00.73
6−0.13
6R4-3
3.44
40.44
4−0.38
6
R5-0
2.30
10.48
00.113
R5-1
3.06
40.39
43.06
4R5-2
3.62
10.55
0−0.37
9R5-3
3.62
20.25
3−0.10
2
R6-0
1.99
80.53
20.02
9R6-1
2.78
20.35
82.78
2R6-2
3.50
80.40
7−0.26
6R6-3
3.64
50.211
0.00
4
R7-0
1.40
10.42
40.02
5R7-1
2.63
40.48
22.63
4R7-2
3.119
0.46
20.10
6R7-3
3.51
90.30
7−0.10
5
R8-0
1.93
00.45
30.12
5R8-1
2.50
70.50
92.50
7R8-2
2.91
40.37
50.28
2R8-3
2.85
40.39
10.14
8
R9-0
2.02
10.52
2−0.03
7R9-1
2.511
0.48
42.511
R9-2
2.84
80.34
40.31
7R9-3
2.83
30.36
40.16
1
R10
-00.28
70.99
9−0.22
1R10
-1–
––
R10
-2−0.96
11.511
0.31
6R10
-32.48
70.41
50.40
4
R11-0
1.25
90.42
30.05
0R11-1
1.25
00.63
41.25
0R11-2
2.08
00.62
6−0.12
4R11-3
2.99
80.39
30.15
6
R12
-0–
––
R12
-1–
––
R12
-2–
––
R12
-33.117
0.66
0−0.16
5
R13
-02.26
00.59
7−0.05
6R13
-12.90
00.39
02.90
0R13
-23.18
50.47
60.05
1R13
-32.86
80.67
4−0.15
2
R14
-02.21
70.50
40.04
3R14
-13.20
00.53
33.20
0R14
-23.24
20.90
4−0.53
1R14
-33.29
90.53
8−0.34
6
R15
-0–
––
R15
-12.91
80.66
22.91
8R15
-22.60
10.81
0−0.21
4R15
-32.57
91.02
9−0.17
6
R16
-01.77
60.73
2−0.17
2R16
-13.31
20.54
63.31
2R16
-23.25
91.01
8−0.63
5R16
-32.10
01.95
4−0.66
8
R17
-01.52
11.62
4−0.49
2R17
-13.05
10.67
93.05
1R17
-22.87
00.93
8−0.41
4R17
-33.49
60.40
7−0.37
3
R18
-02.44
51.03
2−0.52
4R18
-13.40
70.52
63.40
7R18
-22.97
80.94
4−0.44
6R18
-33.39
60.52
2−0.44
1
R19
-02.01
70.40
2−0.22
0R19
-13.20
60.48
53.20
6R19
-22.75
80.61
70.22
3R19
-32.28
10.62
50.03
5
R20
-01.21
80.50
7−0.00
8R20
-11.42
40.62
51.42
4R20
-22.94
81.03
4−0.66
6R20
-33.30
80.71
9−0.54
3
R21
-01.04
10.46
1−0.23
5R21
-12.95
90.93
92.95
9R21
-21.38
90.52
0−0.18
0R21
-31.55
70.89
9−0.20
9
R22
-01.43
40.58
6−0.12
1R22
-11.68
50.80
91.68
5R22
-21.75
90.66
40.01
8R22
-31.74
30.72
00.09
1
R23
-01.33
30.65
0−0.37
8R23
-11.88
60.86
91.88
6R23
-21.55
80.89
60.06
7R23
-32.56
50.51
9−0.07
1
R24
-01.34
70.48
0−0.12
8R24
-11.15
10.71
61.15
1R24
-21.93
90.84
1−0.22
9R24
-31.09
00.75
90.16
2
R25
-01.47
80.42
6−0.27
2R25
-11.87
10.50
41.87
1R25
-22.27
50.51
90.15
2R25
-32.46
20.45
50.21
8
R26
-00.63
70.78
0−0.16
0R26
-1–
––
R26
-2–
––
R26
-3−0.04
90.80
4−0.39
8
Ssamples,M
zmeangrainsize
inØ,σ
sortingindexinØ,Skiskew
nessinØ,R
1-0toR26
-0samples
collected
from
0m,R
1-1toR26
-1samples
collected
from
−2m,R
1-2toR26
-2samples
collected
from
−5m,R
1-3to
R26
-3samples
collected
from
−7m
Arab J Geosci
Miliane River and by a muddy facies (0 %≤FG≤30 %)between Ghar El Melh and Kalâat El Andalous.
– the eastern coastline, which is situated between BorjCédria and Sidi Daoued, is generally covered by onlya sandy facies.
According to the map of the Gulf of Tunis bathymetry andthe hydrographic system (Fig. 2), the abundance of the finefraction in the western coast, between Ghar El Melh andRaoued and between La Goulette and Hammam-Lif, is dueto the important role of the Medjerda and Miliane rivers thatcarry significant quantities of sediment, especially of finefractions. It is also due to the smoothness of the submarineslope in these sectors. The gentle slope dissipates the waves’energy (Mignot 1988; Paskoff 1994), giving rise to fine par-ticle deposits. The increase of the fine fraction (with respect to
the decrease of the coarse fraction) from the coast to the opensea is explained by the ability of fine particles to be mobilized,transported by currents, and finally deposited in the offshore.
The absence of the mixed and muddy facies in SidiAli El Mekki and between Raoued and Salammbo ismainly due to the importance of the submarine slope inthese sectors. At the eastern coast, the lack of sandymud and muddy facies is mainly due to the importanceof the underwater slope. Being steep, shallows favorthe concentration of the waves’ energy (Mignot 1988;Paskoff 1994) and the creation of an agitated environ-ment. Currents of high energy mobilize fine particlesand keep the larger particles in place.
The rarity of the fine fraction is also due to the scarcity ofrivers that feed the Gulf of Tunis in sediments, especiallybetween Port aux Princes and Sidi Daoued. The proportion
Fig. 14 Distribution map of the mean grain size (Mz) of superficial sediments of the Gulf of Tunis
Arab J Geosci
of the coarse fraction of sediments collected from mouths ofrivers (Table 3) decreases, generally moving away from theshoreline and the facies becomes muddy with predominanceof the fine fraction.
Grain size parameters
Mean grain size
The spatial distribution of the mean grain size of super-ficial sediments collected from different depths (Table 4;Fig. 14) shows that the western coast is covered, gen-erally, by fine to very fine sands between Sidi Ali ElMekki and Raoued (2.08 Ø<Mz<3.62 Ø) and betweenGammarth and Borj Cédria (2.13 Ø<Mz<3.35 Ø). Be-ing an exception, the littoral of Gammarth is covered bycoarse sands (−0.96 Ø<Mz<0.28 Ø).
Such distribution may be due to the effect of theMedjerda and Miliane rivers which supply the Gulf ofTunis with sediments, especially with fine particles. On
the other hand, in these sectors, the submarine areas,which are generally gently sloping, dissipate energy andlead to fine particle deposits. However, the existence ofcoarse sands in the littoral of Gammarth is due to theamplification of the waves’ energy in meeting the cliffof Cap Gammarth and the removal of the fine particles.
However, the eastern coast situated between BorjCédria and Sidi Daoued is generally covered by mediumsands betweenBorj Cédria and Sidi Erraïs (1Ø<Mz<1.7Ø) andbetween Port aux Princes and Bir El Jadi (1.15 Ø<Mz<1.93 Ø),with the existence of coarse sands in Sidi Daoued(−0.049 Ø<Mz<0.637Ø). The deposit of medium tocoarse sands in the eastern coast is due to the steepslopes of the submarine area which favor the concen-tration of the energy of the waves.
The variation of the mean grain size of superficialsediments of the Gulf of Tunis (Fig. 15) shows that,along the western coast, the grain size decreases fromSidi Ali El Mekki to Kalâat El Andalous, indicatingthe existence of N–S sediment transport done by
Fig. 15 Variation of the meangrain size (Mz) of superficialsediments of the Gulf of Tunis
Arab J Geosci
longshore currents, especially behind the Sand Spit ofKalâat El Andalous (Fig. 5). The grain size increasesfrom Raoued to Gammarth, indicating a sediment trans-port done by SE–NW longshore currents, from Gam-marth to Raoued (Fig. 5). However, it decreases fromSidi Bou Saïd to La Goulette, confirming the existenceof sediment transports made by NE–SW longshore cur-rents (Figs. 6 and 7), and it decreases from Hammam-Plage to Ezzahra, showing a SE–NW sedimenttransport.
In the eastern coast, there is no evolutionary trend ofthe grain size along the coastline situated between BorjCédria and Sidi Erraïs because of the existence ofgyratory currents and currents of high energy generatedby the dominant NW waves (Fig. 10). The grain sizedecreases from Port aux Princes to Rtiba, showing theexistence of a sediment transport having a SW–NE
direction. It is due to longshore currents (Fig. 11). Itdecreases from Sidi Daoued to Bir El Jadi, confirmingthe existence of a sediment transport done by NE–SWlongshore currents (Fig. 11).
Standard deviation
The spatial distribution of the standard deviation of thesuperficial sediments collected from different depths(Table 4; Fig. 16) shows that the majority of the Gulf ofTunis sands are moderately to well sorted.
The variation of the standard deviation (σ) of thesuperficial sediments of the Gulf of Tunis (Fig. 17)shows that sorting is generally improving, from thecoast to the sea. This sorting is generally due to thedecrease of the grain size of superficial sediments mov-ing away from the shore.
Fig. 16 Distribution map of the standard deviation (σ) of superficial sediments of the Gulf of Tunis
Arab J Geosci
Skewness
The spatial distribution of the skewness of the super-ficial sediments of the Gulf of Tunis (Table 4; Fig. 18)shows that the western coast is generally covered bysymmetrical and highly coarse-skewed sands (−0.668Ø<Ski<0.05 Ø), with the exception of the sectors of Raouedand La Marsa which are covered by fine to highly fine-skewed sands (0.156 Ø<Ski <0.404 Ø). The easterncoast is also covered by symmetrical and coarse-skewed sediments (−0.543 Ø<Ski<0.091), with the ex-ception of sands collected between Port aux Princes andBir El Jadi which are symmetrical to fine-skewed(−0.071 Ø<Ski<0.218 Ø) at the depth of −7 m.
The variation of the skewness of superficial sedi-ments of the Gulf of Tunis (Fig. 19) shows no evolu-tionary trends from the coast towards the sea. It showsthat the majority of sands are symmetrical to highlycoarse-skewed. However, sands that cover the sectorlocated between Raoued and Gammarth are fine-skewed.
Correlation between grain size parameters
Correlation between the mean grain size and the standarddeviation
Sands collected from the small depths are medium tofine and moderately to well sorted between Sidi Ali ElMekki and La Marsa and between Borj Cédria and SidiDaoued. Sands are fine and moderately to well sortedbetween Sidi Bou Saïd and Hammam-Plage (Fig. 20).This correlation shows that finer sediments are charac-terized by a good sorting by the action of hydrodynamicagents (Griffiths 1967; Tucker 1990).
Correlation between the mean grain size and the skewness
The correlation between the mean grain size and theskewness shows that the grain size is a function of theasymmetry: the majority of sands are symmetrical tocoarse-skewed (Fig. 21).
Fig. 17 Variation of thestandard deviation (σ) ofsuperficial sediments of theGulf of Tunis
Arab J Geosci
Correlation between the skewness and the standarddeviation
Sediments of the Gulf of Tunis are, in majority, moderatelyto well sorted and are symmetrical to coarse-skewed(Fig. 22). This is the case of sands for most coastlines inthe world (Tucker 1990).
Conclusion
The determination of the sedimentary facies and the grainsize characteristics of superficial sediments of the Gulf ofTunis lead to dividing this Gulf into two different coasts:
– the western coast which extends from Sidi Ali ElMekki to Borj Cedria: this coast is characterized bythe existence of three sedimentary facies: a sandyfacies throughout the coastline accompanied by asandy–muddy facies (mixed), between Ghar ElMelh and Raoued and between La Goulette andEzzahra, with a muddy facies, adjacent to the mixedone, between Ghar El Melh and Kalâat El Anda-lous. The lateral distribution of these three facies isdue to a sorting done by currents which carry thefine particles from the coast to the open sea. Thewestern coast is generally covered by fine to veryfine sands, moderately to well sorted, and symmet-rical to coarse-skewed. The abundance of the fine
Fig. 18 Distribution map of the skewness (Ski) of superficial sediments of the Gulf of Tunis
Arab J Geosci
fraction between Ghar El Melh and Raoued andbetween La Goulette and Hammam-Lif is due tothe important role of the Medjerda and Milianerivers which carry significant amounts of sediments,especially of fine particles. It is also due to thesmoothness of the underwater slope in these areas,which lead to the dissipation of the waves’ energyand fine particle deposits. The increase of the meangrain size in the littoral of Gammarth giving rise tocoarse sands is due to the concentration of N–Ewaves near the cliff of Cap Gammarth.
– the eastern coast which is situated between Borj Cédriaand Sidi Daoued: this coast is characterized by a sandyfacies only. It is generally covered by medium to coarsesands. Such deposit is due to the steep slopes of thesubmarine area which favor the concentration of theenergy of waves.0.
The interpretation of the grain size results and the hydro-dynamic status of the Gulf of Tunis shows that the westerncoast is submitted to N–E waves which generate longshorecurrents responsible of sediment transport: a N–S sediment
transport from Sidi Ali El Mekki to Kalâat El Andalous,especially behind the Sand Spit of Kalâat El Andalous; aSE–NW sediment transport from Gammarth to Raoued; aNE–SW sediment transport from Sidi Bou Saïd to La Gou-lette; and a SE–NW sediment transport from Hammam-Plage to Ezzahra.
In the eastern coast, there is no evolutionary trend ofthe grain size along the coastline situated between BorjCédria and Sidi Erraïs because of the existence of gyra-tory currents and currents of high energy generated by thedominant NW waves. The grain size decreases from Port auxPrinces to Rtiba, showing the existence of sediment transportdue to SW–NE longshore currents. It decreases from SidiDaoued to Bir El Jadi, showing the existence of a sedimenttransport done by NE–SW longshore currents.
The advantage of this work is that presents a recent grainsize study which characterizes superficial sediments of thewhole sectors of the Gulf of Tunis and describes sedimenttransport using grain size characteristics, the propagation ofdominant waves, and the circulation of generated currents.Sediment transport can be applied to solve many environmen-tal and geological problems, notably the marine erosion.
Fig. 19 Variation of theskewness (Ski) of superficialsediments of the Gulf of Tunis
Arab J Geosci
Fig. 20 Correlation between the mean grain size (Mz) and the standard deviation (σ) of the superficial sediments collected from the small depths of SidiAli ElMekki–LaMarsa (a), Sidi Bou Saïd–LaGoulette (b), Radès–Hammam-Plage (c), Borj Cédria–Sidi Erraïs (d), and Port aux Princes–Sidi Daoued (e)
Arab J Geosci
Fig. 21 Correlation between the mean grain size (Mz) and the skewness (Ski) of the superficial sediments collected from the small depths of Sidi Ali ElMekki–La Marsa (a), Sidi Bou Saïd–La Goulette (b), Radès–Hammam-Plage (c), Borj Cédria–Sidi Erraïs (d), and Port aux Princes–Sidi Daoued (e)
Arab J Geosci
Fig. 22 Correlation between the standard deviation (σ) and the skewness (Ski) of the superficial sediments collected from the small depths of Sidi Ali ElMekki–La Marsa (a), Sidi Bou Saïd–La Goulette (b), Radès–Hammam-Plage (c), Borj Cédria–Sidi Erraïs (d), and Port aux Princes–Sidi Daoued (e)
Arab J Geosci
Acknowledgments We would like thank the journal reviewers, Dr.Jean Paul Barusseau (CEFREM—University of Perpignan Via Domitia,Perpignan, France) and Dr. Mohamed Khalifa (University of Malaya,Kuala Lumpur, Malaysia), for their comments, from which the paperhas greatly benefited. We would also like to thank the members ofCoastal Research Group (GIOC) of the University of Cantabria for theirhelp in the use of the Coastal Modeling System (SMC).
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